1. Field of the Invention
The present invention relates to a method for preparing a polyether antibiotic.
2. Description of the Background Art
Polyether antibiotics can be generally characterized as carboxylic acid ionophores which can be produced by culturing Streptomyces type microorganisms. These polyether antibiotics have a basic structure generally consisting essentially of the elements oxygen, hydrogen and carbon and possibly nitrogen and have a molecular weight in the range of about 300 to about 1800, most often from about 400 to about 1200. They have low solubility in water, are generally soluble in low molecular weight alcohols, ethers and ketones, and have at least one, and usually one or two, carboxylic acid groups. A generally comprehensive review of this class of antibiotics is set forth in Westley, Adv. Appl. Microbiology, 22:177-223 (1977). At least twenty different polyether antibiotics were known at the time the Westley article was written. Since then, additional polyether antibiotics have been discovered.
In the previously noted publication, Westley classified the known polyether antibiotics into four separate classes based on ability of the particular antibiotic to effect the transport of divalent cations and based on the chemical structure of the particular antibiotic. Using these criteria, Westley defined class 1a as those polyether antibiotics which are monovalent polyether antibiotics. In addition, the polyether antibiotics of this class have a generally linear configuration, i.e., the carboxylic portion of the polyether molecule is attached either directly or indirectly to a terminal ring structure. They generally include from about four to about six tetrahydropyran and/or -furan structures and up to six total ring structures. Included in class 1a are the polyether antibiotics monensin, laidlomycin, nigericin, grisorixin, salinomycin, narasin, lonomycin, X-206, SY-1, noboritomycins A & B, mutalomycin, and alborixin.
Class 1b of the polyether antibiotics are defined by Westley as monovalent monoglycoside polyether antibiotics. These polyether antibiotics, as the class name suggests, include a glycoside type structure, more specifically, a 2,3,6-trideoxy-4-O-methyl-D-erythrohexapyranose moiety, which is attached to the polyether molecule such that a non-linear type molecule is formed, i.e., the carboxylic portion of the polyether molecule is attached either directly or indirectly to a non-terminal ring structure or the molecule has a side chain ring structure, e.g., a 2,3,6-trideoxy-4-O-methyl-D-erythrohexapyranose moiety. Generally, the polyether antibiotics of this class contain about six or seven tetrahydropyran and/or -furan structures. Included within class 1b are the polyether antibiotics septamycin, dianemycin, A-204, lenoremycin, carriomycin and etheromycin.
Class 2a as defined by Westley is directed to divalent polyether antibiotics. These antibiotics have a generally linear configuration, may contain from about two to about three tetrahydropyran and/or -furan structures, up to about three total ring structures and no nitrogen atoms. Included within class 2a are the antibiotics lasalocid and lysocellin.
Westley's class 2b of polyether antibiotics is directed to divalent pyrrole ethers and thus, in contrast to the antibiotics of the other classes, the class 2b antibiotics contain one or more nitrogen atoms. Included within class 2b are the polyether antibiotics X-14547, and A-23187 also known as calcimycin.
Polyether antibiotics are generally produced by fermenting a nutrient-containing liquid fermentation medium or broth inoculated with a microorganism capable of producing the desired antibiotic. Suitable liquid fermentation media are generally aqueous dispersions containing sources of assimilable nitrogen and carbon as is known in the art. The fermentation media can also contain a variety of optional ingredients, if desired, such as for example, pH adjustment agents, buffers, trace minerals, antifoam agents, and the like.
Known methods for recovering polyether antibiotics from fermentation broths generally involve complicated and expensive multi-stage solvent extractions and related filtration, chromatography, concentration, and crystallization operations. For example, the procedure to isolate and purify lysocellin first described by Ebata et al. used acetone, n-butanol and methanol (Ebata et al., J. Antibiotics, 28:118-121 (1975)). U.S. Pat. No. 4,033,823 describes an extraction process involving ethyl acetate, acetonitrile, hexane and methanol for recovering lysocellin. Commonly owned U.S. Pat. No. 4,478,935 describes various purified manganese-containing antibiotic complexes extracted from the dried biomass using suitable organic solvents followed by crystallization or precipitation of the complexes. All of these processes follow a rather standard approach in which fermentation broths are subjected to organic solvent extraction to recover the polyether antibiotics. The isolation and purification of polyether antibiotics using extraction methods have been extensively reviewed in Hamill et al., "Polyether Antibiotics" pp. 479-520, J. Chromatogr. Lib., Vol. 15. Antibiotics: Isolation, Separation, and Purification, ed. by Weinstein, M. J. and Wagman, G. H. (1978).
There remains a need in the art for a method for preparing polyether antibiotic material without the need for complicated and expensive multi-stage solvent extractions and related filtration, chromatography, concentration and crystallization operations and the like.